Firefighters doused the charred and smoking entrance to the club, which occupies two basement floors, as paramedics rushed victims into ambulances. Earlier footage showed flames reaching the third of the residential building's 16 storeys.
Broadcaster CNN Turk said fibre materials had caught fire from an explosion that occurred during the sound insulation and decor renovation works in the nightclub. It also said the club had only one exit and that most of those trapped inside died from smoke poisoning.
The blaze broke out at 12:47 p.m. (0947 GMT) in the city's Gayrettepe district, and eight people were detained as part of an investigation, the city governor's office said.
Istanbul Mayor Ekrem Imamoglu, re-elected on Sunday, told reporters outside the building that there had been no application for a permit to renovate or do construction at the site, adding that any work would not have been visible from street level.
The Masquerade website said it would be closed from March 10 to April 10 "due to our new design renovation". It can host up to 4,000 people with DJ performances and stage shows several times a week, it says.
A club representative could not immediately be reached.
Authorities cut power, water and gas to the area and building residents were evacuated as a precaution.
